Implantable cardiac stimulation device including a system for and method of automatically inducing a tachyarrhythmia

ABSTRACT

A system and method for use in an implantable cardiac stimulation device permits automatic induction of a tachyarrhythmia of a heart to permit the performance of an electrophysiological test of the heart. A pulse generator repeatedly delivers a group of first and second sets of pacing pulses to a chamber of the heart. The pacing pulses are separated in time by interpulse intervals to overdrive pace a chamber of the heart. A processor, coupled to the pulse generator, varies the second set of interpulse intervals according to a predetermined protocol after each group of pacing pulses is delivered to the chamber of the heart. The successive groups of pacing pulses are delivered to the heart until the tachyarrhythmia is induced.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. patent applicationSer. No. 09/552,299, filed Apr. 18, 2000, titled “Implantable CardiacStimulation Device Including a System For and Method of AutomaticallyInducing a Tachyarrhythmia,” now U.S. Pat. No. 6,453,197.

FIELD OF THE INVENTION

The present invention generally relates to an implantable cardiacstimulation device. The present invention more particularly relates tosuch a device which includes a system for and implements a method ofautomatically conducting a non-invasive programmed stimulation (NIPS)procedure for inducing a tachyarrhythmia of a heart to permit theperformance of electrophysiological studies.

BACKGROUND OF THE INVENTION

Frequently, a clinician must perform electrophysiological studies inpatients having implanted permanent pacemakers orcardioverters/defibrillators to determine accurately the patient'spathological condition, cardiovascular characteristics and otherinformation needed in order to prescribe a particular therapeutictreatment for the patient. Such studies often require the inducement ofa tachyarrhythmia of the heart such as ventricular tachycardia.

Originally, these studies were invasive because they required thesurgical insertion of instruments such as temporary intracardiac pacingcatheters into the patient. Thus, these studies were accompanied by somerisk and preferably were performed in hospitals. Therefore, thesestudies were not only expensive, but also time consuming and causedpatient anxiety.

In order to avoid medical risks to the patient and hospitalization,non-invasive programmed stimulation (NIPS) procedures have beendeveloped to permit the electrophysiological studies to be performed ina physician's office during routine follow-up visits.

These procedures utilize the implanted cardiac stimulation device and anexternal programmer. By virtue of the presence of a permanentpacing/defibrillation lead associated with the implanted cardiacstimulation device, the need for placement of a temporary intracardiacpacing catheter is eliminated. Typically, NIPS procedures consistgenerally of the application of premature electrical pulses at preciseintervals to the myocardium of the patient's heart by the implantedcardiac stimulation device and its associated lead or leads. Theimplanted device applies the stimulation pulses under commands from theexternal programmer which is manually controlled by the clinician. Priorto the procedure, the clinician manually defines the intervals betweenthe successive stimulation pulses on the programmer. After the implanteddevice applies the succession of stimulation pulses to the heart, theclinician then, on the programmer, manually observes results such astachyarrhythmia induction or lack of capture by the stimulation pulses.Thereafter, the clinician alters the stimulation pulse intervals andcauses the programmer to initiate another application of the successivestimulation pulses to the patient's heart by the implanted cardiacstimulation device. As a result, NIPS studies, as currently constituted,are tedious to administer, consume considerable time, and lead toconsiderable cost to the electrophysiology laboratory.

SUMMARY OF THE INVENTION

The present invention provides a system and method for use in animplantable cardiac stimulation device that automatically induces atachyarrhythmia of a heart to permit the performance of anelectrophysiological test of the heart. In accordance with the presentinvention, the system and method permits the implantable cardiacstimulation device to perform a NIPS procedure automatically onceinitial parameters are established in the device by an externalprogrammer under control of a clinician.

In accordance with the present invention, a NIPS protocol is stored in amemory within the implantable cardiac stimulation device. A processoraccesses the NIPS protocol stored in the memory to control a pulsegenerator that repeatedly delivers groups of pacing pulses to a chamberof the heart. The pacing pulses are separated in time by pacing orinterpulse intervals to overdrive pace the patient's heart. After eachgroup of pacing pulses is delivered to the heart, the processor variesthe pacing intervals according to the NIPS protocol prior toreinitiating the delivery of the next group of stimulation pulses. Theprocessor terminates the delivery of the stimulation pulses if itdetects the tachyarrhythmia of the heart after a group of stimulationpulses have been delivered to the heart.

In accordance with a further aspect of the present invention, theprocessor, in accordance with the NIPS protocol, determines capture ofthe heart by each delivered stimulation pulse and terminates thedelivery of the stimulation pulses to the heart by the pulse generatorwhen a predetermined number of successive pacing pulses fail to capturethe heart. In accordance with a still further aspect of the presentinvention, the processor requires a minimum pacing interval to determinecapture of the heart by a stimulation pulse. In accordance with the NIPSprotocol, the processor terminates the delivery of the stimulationpulses to the heart when a pacing interval falls below the minimumpacing interval.

In accordance with still further aspects of the present invention, eachgroup of stimulation pulses includes a first set of pulses employed tocapture the heart at an overdrive pacing rate and a second set of pulsesto induce the tachyarrhythmia. The processor, in accordance with theNIPS protocol, varies the pacing intervals of the second set of pacingpulses between the application of the groups of stimulation pulses.

In accordance with still further particular aspects of the presentinvention, the second set of pacing pulses includes a last pulse and asecond to the last pulse and the processor decrements the pacinginterval between the last pulse and the second to the last pulseresponsive to the last pulse capturing the heart in accordance with theNIPS protocol. Further, the second set of pacing pulses further includesa third to the last pulse. The processor, in accordance with the NIPSprotocol, decrements the pacing interval between the second to the lastpulse and the third to the last pulse and resets the pacing intervalbetween the last pulse and the second to the last pulse to an initialinterval responsive to the last pulse failing to capture the heart.

In another embodiment, the process may utilize any starting value forthe interpulse interval for the inducing pulses.

In another embodiment, the process may be adjusted in any direction andwill likely be selected based on the starting interpulse interval value.

In another embodiment, the interpulse interval for the overdrive pulsesin the pulse train may be varied after a predetermined time or number ofunsuccessful attempts.

In another embodiment, the determination of the presence of atachycardia may be either a ventricular tachycardia or an atrialtachycardia.

The application of the successive groups of pacing pulses isautomatically continued with intergroup pacing interval adjustment untilthe tachyarrhythmia is detected by the processor. The procedure mayfurther be terminated when manually terminated by the clinician, when apredetermined number of pacing pulses fail to capture the heart, or whena pacing interval falls below the minimum interval required by theprocessor to determine capture of the heart.

BRIEF DESCRIPTION OF THE DRAWINGS

Further features and advantages of the present invention may be readilyunderstood by reference to the following description taken inconjunction with the accompanying drawings, in which:

FIG. 1 is a functional block diagram of a dual-chamber implantedstimulation device illustrating the basic elements of a stimulationdevice which can provide cardioversion, defibrillation, and pacingstimulation, which device embodies and may utilize the present inventionto advantage;

FIG. 2 shows one group of stimulation pulses which may be used forperforming a NIPS procedure in accordance with the preferred embodimentof the present invention; and

FIG. 3 shows a flow chart describing an overview of the operation of thedevice of FIG. 1 in accordance with the preferred embodiment of thepresent invention.

FIG. 4 shows a flow chart describing an overview of the operation of thedevice of FIG. 1 in accordance with an alternative embodiment of thepresent invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following description is of the best mode presently contemplated forpracticing the invention. This description is not to be taken in alimiting sense but is made merely for the purpose of describing thegeneral principles of the invention. The scope of the invention shouldbe ascertained with reference to the issued claims. In the descriptionof the invention that follows, like numerals or reference designatorswill be used to refer to like parts or elements throughout.

In FIG. 1, a simplified block diagram is shown of a dual-chamberimplantable stimulation device 10 embodying the present invention andwhich is capable of treating both fast and slow arrhythmias withstimulation therapy, including cardioversion, defibrillation, and pacingstimulation. While a dual-chamber device is shown, this is forillustration purposes only, and one of skill in the art could readilyeliminate or disable the appropriate circuitry to provide asingle-chamber stimulation device capable of treating one chamber withcardioversion, defibrillation and pacing stimulation.

To provide atrial chamber pacing stimulation and sensing, thestimulation device 10 is shown in electrical communication with apatient's heart 12 by way of an implantable atrial lead 20 having anatrial tip electrode 22 and an atrial ring electrode 24 which typicallyis implanted in the patient's atrial appendage.

The stimulation device 10 is also shown in electrical communication withthe patient's heart 12 by way of an implantable ventricular lead 30having, in this embodiment, a ventricular tip electrode 32, aventricular ring electrode 34, a right ventricular (RV) coil electrode36, and an SVC coil electrode 38. Typically, the ventricular lead 30 istransvenously inserted into the heart 12 so as to place the RV coilelectrode 36 in the right ventricular apex, and the SVC coil electrode38 in the superior vena cava. Accordingly, the ventricular lead 30 iscapable of receiving cardiac signals, and delivering stimulation in theform of pacing and shock therapy to the right ventricle.

While only two leads are shown in FIG. 1, it is to be understood thatadditional stimulation leads (with one or more pacing, sensing and/orshocking electrodes) may be used in order to efficiently and effectivelyprovide pacing stimulation to the left side of the heart or atrialcardioversion and/or defibrillation. For example, a lead designed forplacement in the coronary sinus region could be implanted to deliverleft atrial pacing, atrial shocking therapy, and/or for left ventricularpacing stimulation.

The housing 40 (shown schematically) for the stimulation device 10includes a connector (not shown) having an atrial tip terminal 42 and anatrial ring terminal 44, which are adapted for connection to the atrialtip electrode 22 and the atrial ring electrode 24, respectively. Thehousing 40 further includes a ventricular tip terminal 52, a ventricularring terminal 54, a ventricular shocking terminal 56, and an SVCshocking terminal 58, which are adapted for connection to theventricular tip electrode 32, the ventricular ring electrode 34, the RVcoil electrode 36, and the SVC coil electrode 38, respectively. Thehousing 40 (often referred to as the “can”, “case” or “case electrode”)may be programmably selected to act as the return electrode, or anode,alone or in combination with one of the coil electrodes, 36 and 38. Forconvenience, the names of the electrodes are shown next to theterminals.

The stimulation device 10 further includes a programmablemicrocontroller 60 which controls the various modes of stimulationtherapy including a non-invasive programmed stimulation (NIPS) procedurein accordance with the present invention. As is well known in the art,the microcontroller 60 includes a microprocessor, or equivalent controlcircuitry, designed specifically for controlling the delivery ofstimulation therapy and may further include RAM or ROM memory, logic andtiming circuitry, state machine circuitry, and I/O circuitry. Typically,the microcontroller 60 includes the ability to process or monitor inputsignals (data) as controlled by a program code stored in a designatedblock of memory. The details of the design and operation of themicrocontroller 60 are not critical to the present invention. Rather,any suitable microcontroller 60 may be used that carries out thefunctions described herein. The use of microprocessor-based controlcircuits for performing timing and data analysis functions is well knownin the art. Representative types of control circuitry that may be usedfor embodying the invention include the microprocessor-based controlsystem of U.S. Pat. No. 4,940,052 (Mann et al.), the state-machine ofU.S. Pat. No. 4,712,555 (Sholder) and U.S. Pat. No. 4,944,298 (Sholder).For a more detailed description of the various timing intervals usedwithin the stimulation device and their interrelationship, see U.S. Pat.No. 4,788,980 (Mann et al.). The '052, '555, '298 and '980 patents areincorporated herein by reference. As shown in FIG. 1, an atrial pulsegenerator 70 and a ventricular pulse generator 72 generate pacingstimulation pulses for delivery by the atrial lead 20 and theventricular lead 30, respectively, via a switch bank 74. The pulsegenerators, 70 and 72, are controlled by the microcontroller 60 viaappropriate control signals, 76 and 78, respectively, to trigger orinhibit the stimulation pulses. The microcontroller 60 further includestiming circuitry that controls the operation of the stimulation devicetiming of such stimulation pulses (e.g., pacing rate andatrio-ventricular (AV) delay), as well as keeping track of the timing ofany refractory periods, PVARP intervals, noise detection windows, evokedresponse windows, alert intervals, marker channel timing, etc., that iswell known in the art.

The switch bank 74 includes a plurality of switches for switchablyconnecting the desired electrodes to the appropriate I/O circuits,thereby providing complete electrode programmability. Accordingly, theswitch bank 74, in response to a control signal 80 from themicrocontroller 60, determines the polarity of the stimulation pulses(e.g., unipolar or bipolar) by selectively closing the appropriatecombination of switches (not shown) as is known in the art. An atrialsense amplifier 82 and a ventricular sense amplifier 84 are also coupledto the atrial and ventricular leads 20 and 30, respectively, through theswitch bank 74 for detecting the presence of cardiac activity. Theswitch bank 74 determines the “sensing polarity” of the cardiac signalby selectively closing the appropriate switches, as is also known in theart. In this way, the clinician may program the sensing polarityindependent of the stimulation polarity.

Each sense amplifier, 82 and 84, preferably employs a low power,precision amplifier with programmable gain and/or automatic gaincontrol, bandpass filtering, and a threshold detection circuit, known inthe art, to selectively sense the cardiac signal of interest. Theautomatic gain control enables the device 10 to deal effectively withthe difficult problem of sensing the low frequency, low amplitude signalcharacteristics of ventricular fibrillation.

The outputs of the atrial and ventricular sense amplifiers, 82 and 84,are connected to the microcontroller 60 which, in turn, inhibit theatrial and ventricular pulse generators, 70 and 72, respectively, in ademand fashion whenever cardiac activity is sensed in the respectivechambers.

For arrhythmia detection, the device 10 utilizes the atrial andventricular sense amplifiers, 82 and 84, to sense cardiac signals todetermine whether a rhythm is physiologic or pathologic. As used herein“sensing” is reserved for the noting of an electrical depolarization,and “detection” is the processing of these sensed depolarization signalsand noting the presence of an arrhythmia. The timing intervals betweensensed events (e.g., the P—P and R—R intervals) are then classified bythe microcontroller 60 by comparing them to a predefined rate zone limit(i.e., bradycardia, normal, low rate VT, high rate VT, and fibrillationrate zones) and various other characteristics (e.g., sudden onset,stability, physiologic sensors, and morphology, etc.) in order todetermine the type of remedial therapy that is needed (e.g., bradycardiapacing, anti-tachycardia pacing, cardioversion shocks or defibrillationshocks, also known as “tiered therapy”). In accordance with the presentinvention, the microcontroller 60 may employ a high rate classificationto determine if a tachyarrhythmia is present for terminating the NIPSprocedure when the tachyarrhythmia has been induced by the NIPSprocedure.

Cardiac signals are also applied to the inputs of an analog to digital(A/D) data acquisition system 90. The data acquisition system 90 isconfigured to sense or acquire intracardiac electrogram signals, convertthe raw analog data into a digital signal, and store the digital signalsfor later processing and/or telemetric transmission to an externaldevice 102. The data acquisition system 90 is coupled to the atrial andventricular leads, 20 and 30, through the switch bank 74 to samplecardiac signals across any pair of desired electrodes.

The data acquisition system is preferably coupled to themicrocontroller, or other detection circuitry, for detecting an evokedresponse from the heart 12 in response to an applied stimulus, therebyaiding in the detection of “capture”. As will be seen hereinafter,capture detection is especially desirable in implementing a NIPSprocedure. Capture occurs when an electrical stimulus applied to theheart is of sufficient energy and applied at an appropriate time in acardiac cycle to depolarize the cardiac tissue, thereby causing theheart muscle to contract. The microcontroller 60 detects adepolarization signal during a window following a stimulation pulse, thepresence of which indicates that capture has occurred. Themicrocontroller 60 may enable capture detection by starting a capturedetection window when a ventricular stimulation pulse is issued usingthe timing circuitry within the microcontroller 60, and enabling thedata acquisition system 90 via control signal 92 to sample the cardiacsignal that falls in the capture detection window and, based on theamplitude, determines if capture has occurred. As is known in the art,this form of capture detection requires a minimum interpulse interval tosustain the window detection. Further, capture detection is preferablyperformed on a beat-by-beat basis during a NIPS procedure.

The implementation of capture detection circuitry and algorithms arewell known. See for example, U.S. Pat. No. 4,729,376 (Decote, Jr.); U.S.Pat. No. 4,708,142 (Decote, Jr.); U.S. Pat. No. 4,686,988 (Sholder);U.S. Pat. No. 4,969,467 (Callaghan et al.); and U.S. Pat. No. 5,350,410(Mann et al.), which patents are hereby incorporated herein byreference. The type of capture detection system used is not critical inthe implementation of the present invention.

The microcontroller 60 is further coupled to a memory 94 by a suitabledata/address bus 96, wherein the programmable operating parameters usedby the microcontroller 60 are stored and modified, as required, in orderto customize the operation of the stimulation device 10 to suit theneeds of a particular patient. Such operating parameters define, forexample, pacing pulse amplitude, pulse duration, electrode polarity,rate, sensitivity, automatic features, arrhythmia detection criteria,and the amplitude, waveshape and vector of each shocking pulse to bedelivered to the patient's heart 28 within each respective tier oftherapy. The memory 94 preferably also stores a NIPS protocol from whichthe microcontroller automatically controls the NIPS procedure.

The operating parameters of the implantable device 10, the NIPSprotocol, and the NIPS initializing parameters or values may benon-invasively programmed into the memory 94 through a telemetry circuit100 in telemetric communication with an external device 102, such as aprogrammer, transtelephonic transceiver, or a diagnostic systemanalyzer. The telemetry circuit 100 is activated by a control signal 106from the microcontroller 60. The telemetry circuit 100 also allowsintracardiac electrograms, status information relating to the operationof the device 10 (as contained in the microcontroller 60 or memory 94)to be sent to the external device 102 through the establishedcommunication link 104.

The stimulation device 10 further includes a physiologic sensor 110.Such sensors are commonly called “rate-responsive” sensors. Thephysiological sensor 110 is used to detect and generate a raw signalrepresenting the activity of the patient, to which the microcontroller60 responds by adjusting the rate and AV Delay at which the atrial andventricular pulse generators, 70 and 72, generate stimulation pulses.The type of sensor used is not critical to the present invention and isshown only for completeness.

The stimulation device additionally includes a battery 114 whichprovides operating power to all of the circuits show in FIG. 1. For thestimulation device 10, which employs shocking therapy, the battery mustbe capable of operating at low current drains for long periods of timeand then be capable of providing high-current pulses (for capacitorcharging) when the patient requires a shock pulse. The battery 114 mustalso have a predictable discharge characteristic so that electivereplacement time can be detected. Accordingly, the device 10 may employlithium/silver vanadium oxide batteries, as is common in many suchdevice to date.

The stimulation device 10 further includes a magnet detection circuitry(not shown), coupled to the microcontroller 60. It is the purpose of themagnet detection circuitry to detect when a magnet is placed over thestimulation device 10, which magnet may be used by a clinician toperform various test functions of the stimulation device 10 and/or tosignal the microcontroller 60 that an external programmer 102 is inplace to receive or transmit data, including NIPS procedure initializingparameters to the microcontroller 60 through the telemetry circuits 100or instructions to initiate or terminate a NIPS procedure.

As further shown in FIG. 1, the device includes an impedance measuringcircuit 120 which is enabled by the microcontroller 60 by a controlsignal 122. The known uses for an impedance measuring circuit 120include, but are not limited to, lead impedance surveillance during theacute and chronic phases for proper lead positioning or dislodgment;detecting operable electrodes and automatically switching to an operablepair if dislodgment occurs; measuring respiration or minute ventilation;measuring thoracic impedance for determining shock threshold; detectingwhen the device has been implanted; measuring stroke volume; anddetecting the opening of the valves, etc. The impedance measuringcircuit 120 is advantageously coupled to the switch bank 74 so that anydesired electrode (including the RV and SVC coil electrodes, 36 and 38)may be used. The impedance measuring circuit 120 is shown only forcompleteness.

An important function of the device 10 is to function as an implantablecardioverter/defibrillator (ICD) device. That is, it is capable ofdetecting the occurrence of an arrhythmia, and automatically applying anappropriate electrical shock therapy to the heart aimed at terminatingthe detected arrhythmia. To this end, the microcontroller 60 furthercontrols a shocking circuit 130 by way of a control signal 132. Theshocking circuit 130 generates shocking pulses of low (up to 0.5Joules), moderate (0.5–10 Joules), or high energy (11 to 40 Joules), ascontrolled by the microcontroller 60. Such shocking pulses are appliedto the patient's heart through at least two shocking electrodes, and asshown in this embodiment, using the RV and SVC coil electrodes, 36 and38, respectively. In alternative embodiments, the housing 40 may act asan active electrode in combination with the RV electrode 36 alone, or aspart of a split electrical vector using the SVC coil electrode 38 (i.e.,using the RV electrode as common).

Cardioversion shocks are generally considered to be of low to moderateenergy level (so as to minimize pain felt by the patient), and/orsynchronized with an R-wave and/or pertaining to the treatment oftachycardia. Defibrillation shocks are generally of moderate to highenergy level (i.e., corresponding to thresholds in the range of 5–40Joules), delivered asynchronously (since R-waves may be toodisorganized), and pertaining exclusively to the treatment offibrillation. Accordingly, the microcontroller 60 is capable ofcontrolling the synchronous or asynchronous delivery of the shockingpulses.

While a two chamber implantable stimulation device has been shown anddescribed, the system and method for inducing a tachyarrhythmia may alsobe embodied in a multi-chamber device. For a complete description of animplantable stimulation device capable of pacing and sensing in two tofour chambers and/or providing, anti-tachycardia pacing and shocktherapy, see U.S. application Ser. No. 09/930,725, entitled “Multi-sitecardiac stimulation device for controlling Interchamber Delay” (RichardLu), filed Aug. 14, 2001, which application is incorporated herein byreference.

After either activity block 158 or activity block 162 is completed, themicrocontroller determines if a condition exists to terminate the NIPSprocedure. The first such condition is illustrated by decision block 164wherein the microcontroller determines if there has been a capturefailure. More particularly, in accordance with this embodiment, such acapture failure is a condition wherein a predetermined number ofconsecutive stimulation pulses have failed to capture the heart. Apredetermined number of successive stimulation pulses failing to capturethe heart may be, for example, two consecutive pulses. As previouslymentioned, capture is preferably verified after each stimulation pulse.If capture fails it is noted by the microcontroller. In decision block164, the microcontroller performs an analysis of the notations. If thenotations indicate that two consecutive stimulation pulses failed tocapture the heart, the process immediately returns.

The second set 144 of pulses, commonly referred to as the extra pulses,includes pulse S2, pulse S3, and pulse S4. The number of extra pulsesmay also be a programmable parameter and vary from that describedherein. Each of the S2, S3, and S4 pulses also preferably has aprogrammable amplitude and pulse width of, for example, 4.5 V and 0.5milliseconds, respectively. The last S1 pulse and the extra pulses areseparated in time by interpulse intervals S1-S2, S2-S3, and S3-S4 whichare progressively decreasing. The extra pulse interpulse intervals maybe initially set by the programmer 102 and may be 280 milliseconds, 260milliseconds, and 240 milliseconds, respectively. The intended purposeof the extra pulses is to place the last extra pulse, pulse S4, inaccordance with this embodiment, in an overdriven cardiac cycle of theheart where the stimulated heart tissue is only partially refractory forinducing the tachyarrhythmia.

To place the pulse S4 as described above, the microcontroller 60 inaccordance with this preferred embodiment and the stored NIPS protocol,adjusts or varies the interpulse intervals S2-S3, and S3-S4 in aprescribed manner after each group 140 of pulses is delivered to theheart. After each adjustment of the interpulse intervals, the pulsetrain is reinitiated and delivered to the heart. The foregoing processcontinues until the tachyarrhythmia is induced.

A prescribed manner in which the microcontroller 60 may vary theinterpulse intervals and hence control the performance of the NIPSprotocol of this embodiment is particularly illustrated in FIG. 3. Inthis flow chart, the various algorithmic steps are summarized inindividual “blocks”. Such blocks describe specific actions or decisionsthat are carried out as the algorithm proceeds. Where a microcontroller(or equivalent) is employed, the flow charts presented herein providethe basis for a “control program” that may be used by such amicrocontroller (or equivalent) to effectuate the desired control of thestimulation device. Those skilled in the art may readily write such acontrol program based on the flow charts and other descriptionspresented herein.

The process of FIG. 3 initiates at an activity step 150 whereininitializing parameters are provided to the microcontroller 60 or memory94 via the telemetry circuit 100. It is, of course, assumed that theNIPS protocol to be implemented has already been stored in the memory 94in the same manner. The initializing parameters may be, for example, theinitial, or starting, values for S1-S1, S1-S2, S2-S3, and S3-S4intervals, the amplitude and durations of the stimulation pulses, thetime between successive pulse trains, and the interval by which eachcoupling interval will be adjusted as called for by the protocol. Inalternative embodiments, shown in FIG. 4, the starting value for theinducing pulses may be a programmable value or a last successful value,and the adjustment may take the form of decrementing, incrementing, or“scanning”, for example, in a binary fashion by alternately decrementingand incrementing, or equivalent, as will be described in more detailbelow. Also, at this time, any normal automatic capture back-up pulsefunction is suspended while capture verification is enabled on abeat-to-beat basis.

As shown in FIG. 3, after the initial parameters have been established,the process proceeds to activity step 152 wherein the microcontroller 60causes the pulse generator 72 to deliver the first pulse train or firstgroup of stimulation pulses as illustrated in FIG. 2 to the heart. Afterthe first pulse train has been applied, the microcontroller next, indecision block 154 determines if the tachyarrhythmia has been induced.If the tachyarrhythmia is detected by the microcontroller, the processreturns. However, if not, the process then advances to decision block156. In decision block 156, the microcontroller determines if pulse S4captured the heart. If pulse S4 captured the heart, the processoradvances to activity step 158 to decrement the S3-S4 interpulseinterval, by, for example, 10 milliseconds. If pulse S4 failed tocapture the heart as determined in decision block 156, the processadvances to activity blocks 160 and 162 in that order to decrement theS2-S3 interpulse interval and to reset the S3-S4 interval, if previouslydecremented, to its initial value. The process is now ready toreinitiate another pulse train with the adjusted intervals unless atermination condition exists to be described subsequently. Absence sucha condition, the process will continue to adjust the interpulseintervals as described above after each pulse train. The pulse trainsare reinitiated by the microcontroller until the tachyarrhythmia isdetected in decision block 154.

After either activity block 158 are activity block 162 is completed, themicrocontroller determines if a condition exists to terminate the NIPSprocedure. The first such condition is illustrated by decision block 164wherein the microcontroller determines if there has been a capturefailure. More particularly, in accordance with this embodiment, such acapture failure is a condition wherein a predetermined number ofconsecutive stimulation pulses have failed to capture the heart. Apredetermined number of successive stimulation pulses failing to capturethe heart may be, for example, two consecutive pulses. As previouslymentioned, capture is preferably verified after each stimulation pulse.If capture fails it is noted by the microcontroller. In decision block164, the microcontroller performs an analysis of the notations. If thenotations indicate that two consecutive stimulation pulses failed tocapture the heart, the process immediately returns.

The next termination condition is illustrated by decision block 166.Here, the microcontroller determines if the NIPS procedure has beenmanually terminated by the clinician initiating a command to terminatethe procedure from the external programmer 102. Such a command isreceived through the telemetry circuit 100 of the device 10. If atermination command is given, it will be noted by the microcontrollerupon receipt and will be acted upon when the process reaches decisionblock 166. If such a notation of termination is present when the processreaches decision block 166, the microcontroller will terminate the NIPSprocedure by immediately returning.

Lastly, the final termination condition is illustrated by decision block168. Here, the microcontroller determines if any one of the interpulseintervals has been decremented to such as extent as to be shorter thanthe minimum interval required to support a beat-by-beat capturedetermination. If an interpulse interval is too short, the processimmediately returns. If no pulse intervals is too short, the processreverts to activity block 152 to initiate the delivery of the next pulsetrain.

In FIG. 4, a more generalized embodiment is shown that includes severalfeatures for enhanced flexibility. In one embodiment, for example, theprocess may utilize any starting value (e.g., a maximum value, a minimumvalue, a programmable value, or a last successful value stored inmemory) for the interpulse interval for the inducing pulses (e.g., S2,S3, S4, etc.). In another embodiment, the process may be adjusted in anydirection (e.g., by decrementing, incrementing or any combinationthereof) and will likely be selected based on the starting interpulseinterval value. In another embodiment, the pulse train overdrive pulses,S1, may be varied after a predetermined time or number of attempts(e.g., from 400 ms to 375, from 375 ms to 350 ms, from 350 ms to 325 ms,or any predefined combination of programmable values, etc.).Furthermore, the generalized embodiment of FIG. 4 explicitly shows thatthe determination of the presence of a tachycardia (e.g., steps 253 and262) may be either a ventricular tachycardia or an atrial tachycardia.These features have been combined into a single flow chart shown in FIG.4 for convenience. It is recognized that one of skill in the art couldreadily eliminate, or make minor modifications to one or more of thefeatures, or alter the order of the steps, and not depart from thespirit of the invention. What follows is a generalized description forall of these features.

The process begins at an activity step 250 wherein pacing parameters forS1, S2, S3, and S4 are initialized, including a starting interpulseinterval for S2-S3 and S3-S4, which may be programmable values, e.g., amaximum value, a minimum value corresponding when the tissue is nolonger refractory, or a last successful value that has been stored inmemory. If the latter, then the microcontroller 60 and memory 94 havebeen programmed to store the last successful starting values from aprevious attempt, and may even be further programmed to classify thelast successful starting values according to the VT rate that wasinduced and the morphology of the rhythm. Once the pacing parameters areinitialized, a first group of stimulating pulses, or pulse train, isdelivered to patient's heart as illustrated in FIG. 2.

The process performs automatic capture detection at step 253 on abeat-by-beat basis for each of the pulses (e.g., S1, S2, S3, S4, etc).However, for simplicity of the flow diagram shown in FIG. 4, the step ofdetermining capture is shown as a single step in dotted lines toindicate that it may be performed continuously and in the background.Any time that capture is lost, the process will terminate at step 280,and may also telemeter a message to the external programmer as to whythe test has been terminated. The determination of capture eliminatesthe need for the physician to visually check for capture on an ECGstrip, which can often be difficult to discern. For example, a loss ofcapture during the overdrive pulses, S1, indicates a failure toadequately entrain the heart. Entraining the heart ensures a regularrhythm, and thus a reasonably consistent refractory period, which helpsin obtaining repeatable results. A loss of capture in any of theinducing pulses (e.g., S2, S3, S4, etc.) generally would indicate thatthe interpulse intervals of the inducing pulses are too short andfalling in the refractory period.

In steps 254 and 262, the generalized flow chart of FIG. 4 explicitlyindicates that the tachyarrhythmia determination may be a ventriculartachycardia determination (VT) or an atrial tachycardia (AT)determination. When ventricular tachycardia induction is performed, theprocess utilizes ventricular capture techniques, as described above.When atrial tachycardia induction is performed, the process utilizesatrial capture techniques.

Historically, atrial capture has been challenging due to the smallsignal size of the depolarization signal when compared to the largepolarization signals and other noise. While conventional techniques thatare employed in ventricular capture detection schemes may be used withlow polarization leads for atrial capture, several alternate techniqueshave also been developed in recent years to detect atrial capture.

For example, in U.S. Pat. No. 5,601,615, Markowitz, et al. teaches thatatrial loss of capture (ALOC) in response to an A-pace test stimulus isdeclared by the absence of a detected ventricular depolarization(V-event) in the latter portion of the paced A-V delay intervalfollowing the delivery of the A-pace test stimulus.

In U.S. Pat. No. 5,713,933, Condie, et al. utilize circuitry thatmonitors the cardiac impedance waveform during a predetermined capturedetect window following delivery of stimulating pulses.

In U.S. Pat. No. 6,216,037, Van Oort discloses that “when the pacemakersees a change in QT within the 2–10 ms range, this indicates failure of(atrial) capture, and responsive action is taken by either directlyincrementing the energy level of the atrial pace pulses, or performingan atrial threshold search and then resetting the atrial pace energylevel”.

In U.S. Pat. No. 6,295,471, Bornzin et al. performs the test when thepatient is at or near rest and monitors the patient's P-wave activity todetermine a detection window where a next P-wave is expected to occur.The stimulation device then delivers an atrial pulse prior to the nextdetection window, and monitors the window to determine whether a P-waveoccurs therein. If a P-wave does not occur, then atrial capture ispresent, while occurrence of a P-wave indicates absence of atrialcapture. If atrial capture is absent, the stimulation deviceautomatically determines an appropriate atrial pacing threshold bymonitoring the detection window while adjusting the stimulation pulseenergy level. The '471 patent further employs a “bottom-up” adjustingscheme which starts at a low energy level, below the expected atrialpacing threshold, and increases the energy level until atrial capture isdetected, thus saving energy and further avoiding corruption by largepolarization signals.

The '615, '933, '037, and '471 patents are hereby incorporated herein byreference, however, the type of capture detection system used is notcritical in the implementation of the present invention.

Thus, once the pacing train has been delivered at 252, and capture hasbeen verified in the appropriate chamber (i.e., ventricle or atrium),the process determines if a tachycardia (i.e., VT or AT) is present atstep 254. If Yes, then the process terminates at step 280 and the devicetelemeters that induction has been successful. If no at step 254, thenthe interpulse intervals will be automatically adjusted as the definedby the protocol in steps 260–270. The protocol can be varied initiallyby preprogramming desired starting interpulse values for the inducingpulses and a direction of the adjustment.

For example, in the embodiment in FIG. 3, the starting interpulse valuesfor inducing pulses are set to a maximum value and the interpulseintervals are decremented until capture is lost, thereby indicating thatthe interpulse interval is at a minimum value and the last stimulus fellinto refractory tissue.

In another embodiment, the starting value for S3-S4 may be set to aminimum value (e.g., at or near the end of the refractory period, asdetermined either by testing for a loss of capture, or by programming anapproximate value) and then incrementing the interpulse interval until amaximum value, or limit, is reached.

In yet another embodiment, a “last successful” value may be employed.That is, at implant, a starting value is selected by the physician,based on his own prior knowledge of this patient or a large populationof patients in general (i.e., a best guess). Then, onceelectrophysiological testing has begun, the device can keep track of thepacing intervals that were successful in inducing VT, and repeatsubsequent tests to determine repeatability with the last successfulvalue. Accordingly, once a starting interpulse value has been selected,the adjustment may take the form of decrementing, incrementing, or“alternately scanning” by alternately decrementing and incrementing, forexample, in a binary fashion or equivalent.

With this background, the automatic adjustment of the interpulseintervals, as defined by the protocol in steps 260–270 of FIG. 4, maynow be described.

If the tachyarrhythmia is not detected by the microcontroller in step254, the process then advances to step 260 to adjust the S3-S4interpulse interval in a first direction (i.e., either by decrementingor incrementing one step, depending on the starting value and thedesired protocol) and a subsequent pacing train is delivered. The stepof capture verification (not shown) is again performed on a beat-by-beatbasis in the background, and the process proceeds to the next stepwhenever capture exists.

If the tachyarrhythmia is not detected in step 262, then the processthen advances to step 264 to adjust the S3-S4 interpulse interval in asecond direction (i.e., again by either decrementing or incrementing,depending on the starting value and the desired protocol) and asubsequent pacing train is again delivered. In some instances thedesired protocol may define the first and second directions to be thesame direction, or, in another embodiment, in opposite or alternatingdirections.

The process will repeat at step 266 until a limit is reached for S3–S4,at which time the process determines at step 268 whether a limit hasbeen reached for the adjustment of S2-S3 interval. If the limit has notbeen reached, then the S3-S4 interpulse interval is reset and the S2-S3interpulse interval may be adjusted, and the process repeats byreturning ultimately to step 252. The limits may be determined by a lossof capture, thereby indicating that the stimulation pulse is falling inrefractory tissue. Alternatively, the limits may be programmed in at thetime of induction testing or may be based on a number of unsuccessfulattempts.

In yet another embodiment, the pacing train delivered in step 252 mayalso be varied. As such, an intermediate decision step is presented atstep 284 to determine if the pulse train should be adjusted. Thedetermination may be based on the total time elapsed without successfulinduction or a predetermined number of failed attempts. Morespecifically, if it is now time to adjust the pacing train, then theinterpulse interval of the overdrive pulses within the pacing train maybe automatically adjusted at step 286 (e.g., from 400 ms to 375, from375 ms to 350 ms, from 350 ms to 325 ms, or any predefined combinationof programmable values, etc.) before returning to step 252 wherein theprotocol of varying the inducing pulses is repeated, as described above.

From the foregoing, it may be seen that the present invention provides anew and improved procedure for inducing a tachyarrhythmia of the heartto support an electrophysiological study. The improved procedure allowsflexibility in the starting values and in the direction of theadjustments, while still providing complete automaticity once theinduction protocol is underway. The procedure, such as a NIPS procedure,may be used to induce either a ventricular or atrial tachyarrhythmia.Since the procedure is truly automated, the tachyarrhythmia may beinduced in less time, with less anxiety to the patient, and with reducedclinical cost.

While the invention has been described by means of specific embodimentsand applications thereof, it is understood that numerous modificationsand variations could be made thereto by those skilled in the art withoutdeparting from the spirit and scope of the invention. It is therefore tobe understood that within the scope of the claims, the invention may bepracticed otherwise than as specifically described herein.

1. In an implantable cardiac stimulation device, a system thatautomatically induces a tachyarrhythmia of a patient's heart, the systemcomprising: a pulse generator that generates a series of pacing pulsesseparated in time by pacing intervals to a desired chamber of thepatient's heart, the series having a predetermined number of overdrivepacing pulses followed by a predetermined number of inducing pacingpulses; a memory that stores starting values for the pacing intervals,and that further stores a predetermined direction for adjusting thestarting values; a sensing system that senses when a tachyarrhythmia hasbeen induced; and a control circuit, coupled to the pulse generator andthe sensing system, that automatically adjusts the pacing intervals fromthe stored starting values in accordance with the predetermineddirection when a tachyarrhythmia has not been induced, and thenautomatically triggers the pulse generator to deliver a subsequentseries of pacing pulses using the adjusted pacing intervals.
 2. Thesystem of claim 1, wherein: the control circuit is configured toautomatically terminate the delivery of the series of pacing pulses tothe patient's heart when a tachyarrhythmia has been induced.
 3. Thesystem of claim 1, further comprising: a detector that detects thepresence or absence of capture of the patient's heart by each deliveredpacing pulse; and wherein the control circuit terminates the delivery ofthe pacing pulses to the patient's heart when capture is not detected ina predetermined number of pacing pulses.
 4. The system of claim 1,wherein: the inducing pacing pulses comprises a last pulse (S_(N)) and asecond-to-the-last pulse (S_(N-1)); and the control circuit adjusts thepacing interval between the second-to-the-last pulse and the last pulse(S_(N-1) to S_(N)) in accordance with the predetermined direction untila predetermined limit is reached.
 5. The system of claim 4, wherein: theinducing pacing pulses further comprises a third-to-the-last pulse(S_(N-2)); and the control circuit adjusts the pacing interval betweenthe second-to-the-last pulse and the third-to-the-last pulse (S_(N-2) toS_(N-1)) and resets the pacing interval between the second-to-the-lastpulse and the last pulse (S_(N-1) to S_(N)) to the starting value whenthe pacing interval between the second-to-the-last pulse and the lastpulse (S_(N-1) to S_(N)) reaches the predetermined limit.
 6. The systemof claim 4, wherein: the starting value is a minimum pacing intervalthat corresponds to non-refractory tissue; and the control circuitincreases the pacing interval from the minimum starting value to amaximum finishing value.
 7. The system of claim 4, wherein: the startingvalue is a predetermined maximum pacing interval; and the controlcircuit decreases the pacing interval from the maximum starting value toa minimum finishing value.
 8. The system of claim 4, wherein: thestarting value is a programmable pacing interval; and the controlcircuit sweeps the pacing interval through a range of intervals in atleast one predetermined direction beginning with the programmed pacinginterval.
 9. The system of claim 4, wherein: the starting value is alast successful value that induced tachycardia; and the control circuitsweeps the pacing interval through a range of intervals about the lastsuccessful value using at least one predetermined direction.
 10. Thesystem of claim 1, wherein: the overdrive pacing pulses comprises aplurality of pulses (S₁) that are delivered at a pacing interval (S₁-S₁)that overdrives the patient's intrinsic rhythm; and the control circuitautomatically adjusts the pacing interval (S₁-S₁) between the pluralityof pulses (S₁) when either one of a predetermined number of unsuccessfulattempts have been made or a predetermined time has elapsed.
 11. Thesystem of claim 1, wherein: the pulse generator generates the series ofpacing pulses to the patient's atrium; and the control circuitautomatically adjusts the pacing intervals when an atrialtachyarrhythmia has not been induced, and then automatically triggersthe pulse generator to deliver a subsequent series of pacing pulses tothe atrium using the adjusted pacing intervals.
 12. The system of claim1, further comprising: a telemetry circuit coupled to the controlcircuit; and an external device that communicates with the telemetrycircuit and displays values for the pacing intervals after eachadjustment by the control circuit.
 13. In an implantable cardiacstimulation device, a system that automatically induces atachyarrhythmia of a patient's heart, the system comprising: stimulationmeans for inducing a tachyarrhythmia in a desired chamber of a patient'sheart using a series of pacing pulses separated in time by predeterminedpacing intervals, the series having a predetermined number of overdrivepacing pulses followed by a predetermined number of inducing pacingpulses; means for defining starting values for the pacing intervals anda direction for adjusting the starting values; determining means fordetermining when a tachyarrhythmia has been induced; adjusting means forautomatically adjusting the pacing intervals between the inducing pacingpulses from the starting values in accordance with the defined directionwhen a tachyarrhythmia has not been induced; and control means forautomatically triggering the stimulation means to deliver a subsequentseries of pacing pulses using the adjusted pacing intervals when atachyarrhythmia has not been induced.
 14. The system of claim 13,wherein: the control means further comprises means for terminating thedelivery of the series of pacing pulses to the patient's heart when atachyarrhythmia has been induced.
 15. The system of claim 13, furthercomprising: capture determining means that determines one of capture andloss of capture of the patient's heart in response to each deliveredpacing pulse; and wherein the control means further comprises means forterminating the delivery of the series of pacing pulses upon loss ofcapture.
 16. The system of claim 13, wherein: the inducing pacing pulsescomprises a last pulse (S_(N)) and a second-to-the-last pulse (S_(N-1));and the adjusting means adjusts the second-to-the-last pulse and thelast pulse (S_(N-1) to S_(N)) pacing interval in accordance with thedefined direction until a predetermined limit it reached.
 17. The systemof claim 16, wherein: the inducing pacing pulses further comprising athird-to-the-last pulse (S_(N-2)); and the adjusting means adjusts thepacing interval between the second-to-the-last pulse and thethird-to-the-last (S_(N-2) to S_(N-1)) pulse interval and resets thesecond-to-the-last pulse and the last pulse (S_(N-1) to S_(N)) pacinginterval to the respective starting value when the (S_(N-1) to S_(N))pacing interval reaches the predetermined limit.
 18. The system of claim13, wherein: the starting value for the pacing intervals is one of amaximum value, a minimum value that corresponds to non-refractorytissue, a programmable value, and a last successful value.
 19. Thesystem of claim 13, wherein: the defined direction for adjusting thestarting values of the pacing intervals is one of an increase, adecrease, or an alternating pattern of increasing and decreasing by atleast one step.
 20. The system of claim 13, wherein: the stimulationmeans comprises means for inducing a tachyarrhythmia in the patient'satrium; and the adjusting means automatically adjusts the pacingintervals when an atrial tachyarrhythmia has not been induced, and thenautomatically triggers the pulse generator to deliver a subsequentseries of pacing pulses to the atrium using the adjusted pacingintervals.
 21. The system of claim 13, further comprising: means fordisplaying the pacing intervals; and telemetry means for transmittingthe pacing intervals after each adjustment by the adjusting means to thedisplay means.
 22. In an implantable cardiac stimulation device, amethod of inducing tachyarrhythmias in a patient's heart, the methodcomprising: generating a series of stimulation pulses to the patient'sheart, the series having at least one overdrive stimulation pulsefollowed by at least two inducing stimulation pulses, the stimulationpulses being separated in time by interpulse intervals; defining aninduction protocol for adjusting the interpulse intervals, the inductionprotocol comprising a starting value for the interpulse intervals and adirection for adjusting the starting value; determining when atachyarrhythmia has been induced; automatically adjusting the interpulseintervals in accordance with the induction protocol whenever the appliedseries of stimulation pulses fails to induce the tachyarrhythmia; andautomatically triggering a subsequent series of stimulation pulses usingthe adjusted interpulse intervals in response to the failure to inducethe tachyarrhythmia.
 23. The method of claim 22, further comprising:terminating the generation of the series of stimulation pulses to thepatient's heart in response to a determination that a tachyarrhythmiahas been induced.
 24. The method of claim 22, further comprising:determining capture of the patient's heart by each applied stimulationpulse.
 25. The method of claim 24, further comprising: terminating thegeneration of the series of stimulation pulses to the patient's heartwhen a predetermined number of stimulation pulses fail to capture thepatient's heart.
 26. The method of claim 24, wherein: the adjusting stepcomprises automatically adjusting the interpulse intervals of theinducing stimulation pulses of each series in accordance with theinduction protocol.
 27. The method of claim 26, wherein the inducingstimulation pulses comprises a last pulse (S_(N)) and asecond-to-the-last pulse (S_(N-1)), wherein: the adjusting stepcomprises adjusting the interpulse interval between thesecond-to-the-last pulse and the last pulse (S_(N-1) to S_(N)) until apredetermined limit is reached.
 28. The method of claim 27, wherein theinducing stimulation pulses comprises a last pulse (S_(N)), asecond-to-the-last pulse (S_(N-1)), and a third-to-the-last pulse(S_(N-2)), wherein: the adjusting step comprises adjusting theinterpulse interval between the third-to-the-last pulse and thesecond-to-the-last pulse (S_(N-2) to S_(N-1)), and adjusting theinterpulse interval between the second-to-the-last pulse and the lastpulse (S_(N-1) to S_(N)) to the starting interval in response to thelast pulse (S_(N)) failing to capture the patient's heart.
 29. Themethod of claim 22, wherein: the defining step comprises defining thestarting value to one of a minimum value that corresponds tonon-refractory tissue, a maximum value, a programmable value, and a lastsuccessful value.
 30. The method of claim 22, wherein: the defining stepcomprises defining the direction for adjusting the starting values ofthe pacing intervals to one of an increase, a decrease, or analternating pattern of increasing and decreasing by at least one step.31. The method of claim 22, wherein: the defining step comprisesdefining an induction protocol for inducing a tachyarrhythmia in thepatient's atrium.
 32. The method of claim 22, further comprising:transmitting current interpulse intervals when adjusted by the adjustingmeans; and displaying the current values for the interpulse intervals onan external device.